Recently, polymer gels, especially those stimuli-responsive "intelligent" gels, have attracted much attention for their potential biological and medical applications, i.e., as special adsorbents, as actuators, and in drug delivery devices. Y. Osada & S. B. Ross-Murphy, Sci. Amer. 268(5):82 (1993) ; BIOLOGICAL AND SYNTHETIC POLYMER NETWORKS (O. Kramer ed., Elsevier, London 1988); M. Shibayama & T. Tanaka, Adv. Polym. Sci. 109:1 (1993); T. Tanaka, I. Nishio, S. Sun, & S. Ueno-Nishio, Science 218:467 (1982); A. S. Hoffman, J. Controlled Release 6:297 (1987). A gel is a three-dimensional cross-linked polymer network swollen by a large quantity of solvent, whose properties fall between those of a liquid and those of a solid. For this reason, polymer gels are sometimes classified as wetware in order to distinguish them from software and hardware.
Despite a variety of interesting physical and chemical properties, synthetic gels are still used mainly in only a few applications, such as in foods, as water adsorbents, and in soft contact lenses. Two main obstacles--namely, the gel shrinking/swelling speed and strength--have limited many of the potential applications of gels. Much effort has been applied in the last two decades to overcome these two obstacles. For example, it has been reported that the slow shrinking rate can be attributed to the formation of a hard skin layer at very initial stage of the gel shrinking, which prevents further diffusion and outflow of solvent molecules, such as water molecules, from inside the gel. A. Gutowska et al., J. Controlled Release 22:95 (1992); T. G. Park & A. S. Hoffman, Enzy. Microb. Technol. 15:476 (1993); K. Sekimoto, Phys. Rev. Lett. 70:4154 (1993); H. Yu & D. W. Grainger, J. Appl. Polym. Sci. 49:1553 (1993); R. Yoshida et al., J. Biomat. Sci.-Polym. Ed. 6:585 (1994); Y. H. Kim et al., J. Controlled Release 28:143 (1994). Using another hydrophobic polymer to modify thermal sensitive polymer gels via an interpenetrating polymer network ("IPN") structure has also been reported. A. Gutowska et al., Macromolecules 27:4167 (1994); R. Yoshida et al., Nature 374:240 (1995). This leads to a stronger gel, and in some cases, a moderately improved shrinking rate. Most of the applications of these IPNs are in drug delivery. Recently, Yoshida et al. reported that grafting a hydrogel can increase its shrinking rate. R. Yoshida et al., Nature 374:240 (1995). However, preparation of such a comb-type grafted hydrogel is not a simple task.
It would therefore be desirable to provide an easily producible polymer gel composition which has improved shrinking rate and strength characteristics. The present invention meets these and other needs.